1. Field of the Invention
This invention relates to a two-wire control system, and more specifically to a method and apparatus for isolating power distribution transmission lines from the electrical noises generated by the power source and electrical loads, thereby permitting the reliable transmission and reception of other signals during that time, and concurrently when the lines are isolated from the loads the invention, for permitting the status of the isolated electrical loads to be evaluated, irrespective of their status.
2. History of the Prior Art
The major problem associated with power line transmission Data Transmission (PLDT) and Power Line Carrier (PLC) is the simultaneous transmission of both power and signal. A typical alternating current (AC) power transmission line has a low line impedance which may vary from one to ten ohms. In addition, such lines are used to supply varying amounts of load current. However, such power transmission lines also contain the higher frequency electrical signal interference that is generated by the source and loads.
In contrast, a typical signal transmission line is set up to achieve a reliable transfer of information by obtaining the highest signal-to-noise ratio possible which occurs when the receiving amplifiers are terminated properly, and the transmitting amplifiers transmit sufficient signal power to overcome the unwanted electrical interferences.
There are two different approaches employed by PLDT and PLC to combine signal and power. One approach is to position the communication device parallel to the source and load. This parallel approach is used when a large amount of power must be transmitted because the communication device does not interfere with the low source to load line impedance. For example, U.S. Pat. No. 4,636,771, issued to Garrold W. Ochs, employed an interface circuit to effect high bridging impedances to the communication signals. U.S. Pat. No. 4,697,166, issued to Thomas J. Warnagiris et al, coupled signals by using a resonate circuit having a low loss transmitter and a high impedance receiver. U.S. Pat. No. 4,766,414, issued to Kenneth C. Shuey, provided a means for electronically tuning the circuit so that it provides a low impedance circuit path, and U.S. Pat. No. 4,885,563, issued to Richard A. Johnson et al, employed a buffer to provide a high receiving impedance and a low output drive impedance. Each of the above used a different method to modify the parallel transmitting and receiving impedances to increase the signal-to-noise ratio of their communication device.
The second method employed by PLDT and PLC to combine the fields couples the communication device in series with the source. The line and load are then coupled in series to the communication device. U.S. Pat. No. 4,592,069, issued to Robert J. Redding, employed a circuit with low resistance to direct current and high impedance for alternating signals. U.S. Pat. No. 4,949,359, issued to Jean-Pierre Voillat, used the digital signals emitted by the master station to feed power to slave stations. The standard power feed of chain repeaters for submarine cables is similar to U.S. Pat. No. 4,949,359 in that a limited amount of current is provided (under 0.5 amperes) and a source supply voltage of over 1,000 volts DC is used.
There are several disadvantages to each of the above approaches. The series approach to communication systems has limited uses because of the fixed load power requirements and the higher fixed line impedance which relates directly to the frequency response of that line. If a chain repeater has a 1 uF capacitance load at the receiving terminal and no electronic aids to boost the frequency response, the amplitude of the signal at the receiver would be less than 63% of the original transmitted signal for all frequency above 500 Hz. Thus, the series communication systems are typically used in specialized situations and require carefully designed circuits that do not change their power requirements as they age.
In addition, all PLDT and PLC devices, using either the series or the parallel method, must employ low DC/high AC impedances, high bridge impedances to communications signals, resonate circuits, electrically tuned circuits, high receiving impedances/low drive impedances, digital signals to feed power, or a constant-current source to minimize the effect of typical power transmission electrical loads and to minimize the low line impedance effects on the signal transmission.
Furthermore, accurate measurement of various line and load parameters would be very beneficial. However, the measurements must be obtained over the electronic circuits and should be repeatable at any time. In addition, a sensing device must be connected to the electronic circuit to perform the measurements. Nevertheless, if possible, the sensing device should not influence or cause any change in the component electrical characteristics which would alter the measured values. When simple sensing devices are used to accurately measure electrical loads such as resistance, capacitance and inductance, these components are isolated from all other components to eliminate any loading which would affect the measurement of the sensing device. When sense devices are used to measure electrical loads in remote locations, or where the load is an integral part of a larger system, a method is required wherein the impedance of the sense device will not affect the system operation. The method should also calculate the actual value of the load in question from the measured impedances, determine whether the load is active and determine whether the load is receiving current from the power source. Finally, the associated electrical noises generated by the source and other loads must be removed.
In summary, the simplest and most reliable measurements are obtained when the sense device does not effect the component under test and the unit under test is completely isolated from all other components and electrical interferences.